Astronomers at UC Berkeley have discovered the companion star of Supernova 1993J, confirming the first Type IIb binary star system using NASA’s Hubble Space Telescope.

Campus astronomy professor Alex Filippenko, postdoctoral researcher Ori Fox and team member Azalee Bostroem of the Space Telescope Science Institute discovered that SN 1993J has a companion star that took away mass and hydrogen from the exploding primary star. Initially discovered in 1993, the supernova could not be fully analyzed until 2012 because of its excessive glare and luminosity.

“The companion star is important because without it, the primary star (the one that eventually becomes a Type IIb supernova) would not have a way to get rid of most of its hydrogen envelope,” Filippenko said in an email. “The companion star can be thought of as a robber, stealing hydrogen from the primary star. So, it’s as though we witnessed a cosmic crime scene!”

The supernova is classified as a Type IIb supernova because it exhibits two types of supernovae and represents the transition between massive hydrogen-rich stars and those that lost their hydrogen envelope — an invisible region of hydrogen — near the end of their lives. While the supernova loses most of its hydrogen envelope, not all of it is lost.

“It’s really weird because of our understanding of how stars either lose entire envelopes or nothing at all,” Fox said, referencing single-star models. “With the binary star model, the companion star takes the outer hydrogen envelope from the primary star and puts it on itself.”

Although the supernova was discovered almost two decades earlier, astronomers were unable to test the theory of a binary star system, because most Type IIb supernovae rarely occur, and the companion star was too dim for astronomers to see.

After acquiring the ultraviolet-capable Hubble, Filippenko and Fox studied the ultraviolet spectrum of the supernova, ultimately finding an ultraviolet source, confirming the binary scenario that was predicted two decades ago.

“This allowed us to see, for the first time, the tell-tale signature of the hot companion star,” Filippenko said in an email.

According to Fillippenko, ground-based telescopes were ineffective because they captured optical images but not ultraviolet wavelengths, which are blocked by the atmosphere’s ozone. The Hubble allowed them to detect the ultraviolet light emitted by the suspected hot companion star.

Shortly after Filippenko and Fox’s discovery, another group of researchers discovered that Supernova 2011dh also has a companion star, increasing the amount of confirmed Type IIb binary companions to two.

“Its a lot of fun — we beat them to the punch by just a few weeks,” Fox said. “It’s healthy competition, and you need that because it drives science. It keeps it an active field.”